As far ahead as you want - the farther in advance you make it, the longer it has to dry. After your piece has dried thoroughly, put it somewhere where it's away from dust and light, but not in a airtight container (a cardboard box is ideal). Exposure to light can make some vegetable colors fade.
I do any gum paste modelling at least 3 days in advance depending on what it is. Figures can take longer to dry depending on size. I have a work room for all my modelling and keep them in there as said before away from light, heat and humidity. Silcone mats are great for standing the finished products on as they don't stick to the surface.
Bear in mind that if you use liquid colours instead of gel colours to colour your gumpaste it may take longer to dry out. 
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